Class D Amplifiers
The output devices are switched on and off at a very high frequency (compared to the desired output signal) and are driven with a PWM signal (Pulse Width Modulation). This creates an output signal that is a square wave with a varying duty cycle. The signal passes through a low-pass filter to remove the high frequency signal. Since the output devices are fully on or fully off, the power dissipation is very low, and efficiency of the output stage can exceed 90% at full power in some cases. Sometimes called a “Digital Amplifier” (usually a misnomer since a square wave is still an analog signal).
Due to continuous advancement in semiconductor technology, class D amplifiers, once an oddity, are now thriving in the market. Driven by the desire for smaller Hi-Fi (and not so Hi-Fi) systems and better efficiency, this segment will continue to grow for the foreseeable future.
Many times class D amplifiers are called “digital amplifiers.” I prefer to call them “switching amplifiers.” There are some varieties that take a digital PCM signal and convert it directly to PWM, so technically it is not inaccurate, though it would in my opinion be technically inaccurate to call an analog input class D amplifier “digital.” Nevertheless people still advertise “digital” amplifiers… because “digital” has been marketed to mean “better than analog” in everything from cell phones to cable to satellite TV to HDTV to HD radio. “Better for whom?” is the question.
Regardless, the current crop of IC class D amplifiers are very good performers, in terms of output power, efficiency, output noise, low distortion (rivaling good Class B amplifiers), and reliability. There are also some high end designs that are even better. With most class D amplifiers though, you have to watch out for changes in high frequency response. This is due to the output filter, generally a second order filter. The output filter is designed to be flat with a particular load impedance. If a lower impedance is used the filter will be “overdamped” and there will be drop in high frequency output. Conversely, if a higher impedance load is used the filter will be “underdamped” and there will be a rise in the high frequency response. The response variation is generally not to severe, but if the amplifier is not loaded the output filter can “ring.” Generally this is planned for, and doesn’t present a hazard to the amplifier.
Class G Amplifiers
This type of amplifier is basically a Class B amplifier with a second voltage rail. When the signal is high enough the higher voltage rail is used, and when it is not needed, the lower voltage rail is used. Due to the dynamic nature of music, this can reduce power dissipation over the Class B amplifier.
Probably more of these amplifiers are used in Pro Audio than home audio, and the improvements in class D technology have pretty much rendered these amplifiers obsolete, in my opinion. Class B (or AB if you must) might be next on the chopping block as Class D designs gain more acceptance.
Class H Amplifiers
This is similar to Class G except only one voltage rail is used, but the voltage of the rail is varied up and down with the input signal. Generally this requires a switch mode power supply (otherwise there would be no point to using Class H, as a linear regulator would dissipate just as much power, if not more, than the Class B amplifier itself).
BASH amplifiers as well as the Carver designs fit into this category.
Any of the above amplifier types can be bridged (a BASH amplifier by design is always bridged). To bridge a stereo amplifier the phase of the input to one of the channels is inverted and the speaker load is driven by both amplifier channels simultaneously. The positive speaker lead is driven by the noninverted channel and the negative speaker lead is driven by the inverted amplifier channel. The ground connection (or common connection) is not used. There is more than one way to accomplish this, but the concept behind running a bridged amplifier is to create a “more” powerful amplifier. I put “more” in quotes because the power supply and/or the output devices will ultimately determine how much power you can get. If the power supply can’t deliver the required current, or if the output devices can’t handle the added stress, then the power output will be limited, and it’s possible that the output devices could be damaged.
If under normal circumstances an amplifier is delivering 1Vrms to an 8 ohm loudspeaker, the speaker receives V2/R = 1/8 Watt of power. If on the other hand the same amplifier is connected in a bridged arrangement, and the same 1Vrms is available at the noninverting output, there will be -1Vrms at the inverting output (or 1Vrms with an inverted phase, or 180° out of phase – pardon my nomenclature for the moment, it’s just to make the math easier). The potential difference (voltage) at the speaker is now 1V - (-1V) = 2V. 2Vrms on an 8 Ohm speaker will now deliver V2/R = 4/8 = 1/2 Watt of power, or 4 times the power. The voltage is doubled, and by Ohm’s law the current is doubled; Volt x Amps = Power and 2 x 2 = 4. But notice again that the current is doubled, and unless the amplifier can deliver that current, that level of power will not be achieved. The doubling of current is why many 4-8 Ohm rated amplifiers can only be used with 8 Ohm speakers (or higher) when operating in bridge mode; the doubling of current with a 4 Ohm load would exceed the design rating of the output devices, or the amplifier could overheat.